<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    var age = 18

    console.log(age)
    var str = '123'
    console.log(str)
    var flag = true
    var empty = undefined
    var obj = null

    console.log( typeof age )  // number
    console.log( typeof str )  // string
    console.log( typeof flag )  // boolean
    console.log( typeof empty )  // undefined

    console.log( typeof obj )  // obejct

    console.log( typeof(age) ) // number
    console.log( typeof(flag) ) // boolean
  </script>
</body>
</html>